The inhabitants of besieged eastern Aleppo have fewer than 10 days to receive aid or face starvation and death from a lack of medical supplies, according to the head of the Syria Civil Defence, or White Helmets.
The volunteer group, which works in opposition-held territory and has rescued thousands of people from buildings bombed in the civil war, is also running out of basic equipment from lorries to diesel and gas masks.
"You cannot imagine how the situation is," Raed al-Saleh told Reuters news agency.
Saleh was in Stockholm to receive the Right Livelihood Award, known as Sweden's Alternative Nobel Prize.
"Doctors and the rescue workers in Aleppo are just using what's left of the equipment after bombardments to do whatever they can do," Saleh said.
Anti-government fighters in the eastern part of Aleppo city have agreed to a United Nations plan for aid delivery and medical evacuations, but the UN is awaiting a green light from Russia and the Syrian government, Jan Egeland, the UN humanitarian adviser, said on Thursday.
Freezing winter conditions
With freezing winter conditions setting in, about 275,000 people are trapped in eastern Aleppo, where the last UN food rations were distributed on November 13.
Saleh said doctors were so short of supplies that they were resorting to making life and death decisions over who receives surgery.
"They cannot accept everyone ... There are not enough materials and not enough doctors," he said.
Saleh said the White Helmets had lost 50 percent of their equipment in the past two months.
"We have consumed all the stock of first aid kits in our centres and we have consumed all our stock of gas masks," he said.
"We are concerned that within 10 days we may consume all our remaining stock of diesel which is required for the ambulances and the trucks to move."
There has been, heavy, continuous and violent shelling on neighborhoods in eastern Aleppo in the last 10 days.
Al Jazeera's Amro Halabi, reporting from Aleppo's Thahrat Awad neighbourhood, said: "Here, a parachute bomb containing toxic material was dropped. It caused multiple cases of asphyxiation among civilians.
"A variety of weapons were used for the shelling including parachute bombs, bombs containing toxic material and barrel bombs that have taken the lives of dozens of civilians.
"The shelling continues in what is the most violent spell of bombing the besieged city has experienced."
The White Helments' Saleh said his workers had responded to approximately 10 chlorine attacks in Aleppo over the past 10 days, the last being on Wednesday.
Rami Abdulrahman, director of the UK-based Syrian Observatory for Human Rights, which monitors the Syrian conflict, said the observatory had documented two incidents of chlorine attacks in the past fortnight.
Banned toxic agents
On November 11, the executive body of the global chemical weapons watchdog the Organisation for the Prohibition of Chemical Weapons (OPCW) condemned the alleged use of banned toxic agents by the Syrian government and the Islamic State of Iraq and the Levant (ISIL) group.
A 13-month international inquiry by the OPCW and UN concluded in a series of reports that Syrian government forces were responsible for the use of chlorine barrel bombs against civilians.
Syrian authorities deny having used chemical weapons in the conflict.
ISIL, also known as ISIS, has not commented.
Saleh also criticised Russia, which is backing Syrian President Bashar al-Assad in the conflict.
"I believe the withdrawal of Russia from the International Criminal Court is because it knows it commits war crimes in Syria and it doesn't want to be accountable," he said.
The White Helmets shared the Right Livelihood award this year with Mozn Hassan, the Egyptian feminist and human rights activist, Russia's Svetlana Gannushkina, who campaigns for the rights of migrants and refugees, and the Turkish newspaper Cumhuriyet.
The four laureates share a cash award of three million Swedish crowns ($325,000).